This invention relates to pressure transducers, and more particularly to differential pressure transducers of the capacitive type that utilize isolation diaphragms.
Capacitive pressure transducers of the present state of the art utilize a sensing diaphragm having at least one electrode on its surface, together with another electrode structure in facing relationship to the electrodes on the sensing diaphragm. As pressure variations flex the sensing diaphragm, the distance between the diaphragm electrode and the opposite electrode changes, providing a capacitance variation which is a measure of the pressure variation that caused the deflection. In a differential pressure transducer, the pressures act on opposite sides of the diaphragm and the deflection is representative of the difference between the the pressures. However, although the differences may be small the absolute line pressures may be large, so that even a very brief exposure to one pressure without the other can result in shattering or excessive deformation of the sensitive diaphragm. For this and other reasons, differential pressure transducers are manufactured in configurations in which there is a backup surface adjacent one or both faces of the diaphragm, so as to limit its excursion by causing it to bottom out within its elastic limits. Alternatively or additionally, isolation diaphrams are interposed to communicate pressures to the opposite sides of the sensing diaphragm. Input pressures exerted by fluid on each side do not act directly on the sensing diaphragm, but act against the outsides of the two isolation diaphragms. The isolation diaphragms then seek to deflect against a liquid that fills the interior space between the isolation diaphragms and the sensing diaphragm, thus communicating the pressures to the sensing diaphragm.
The present state of the art of differential pressure transducers has reached a high level of precision and capability, but users are demanding even more in terms of cost and performance. A number of conflicting requirements must be met in attaining higher levels of effectiveness. For a differential pressure transducer to be highly sensitive, the sensing diaphragm must be precisely deflectable with very small variations, with high repeatability and very low hysteresis. The space between the electrode on the sensing diaphragm and the opposed electrode of the capacitor pair is often very small, so that minute deflections must be accurately measured. The sensing diaphragm and the structure must nonetheless be sufficiently rugged to withstand vibrations and shocks encountered in industrial usage. However, many factors operate to diminish accuracy, linearity and the operative integrity of the unit in general. For example, even though a differential pressure transducer using isolation diaphragms is completely filled with a pressure transducing fluid, the fluid is not truly incompressible, and its dielectric constant changes with the pressure exerted upon it. Similarly, temperature variations cause changes in the dielectric constant, and such changes alter the capacitance reading. The isolation diaphragm is deflectable, but if deflected over a substantial distance its characteristic is not linear, and this also affects the accuracy of the reading, especially if the isolation diaphragm is relatively stiff, or in the range of 10%-30% of the sensing diaphragm stiffness.
Moreover, in the prior art constructions the isolation diaphragms have been fabricated to special curvatures and convolutions, with concentric waves shaped in accordance with complex calculations. Such curvatures are employed because pressure variations with current designs are otherwise transmitted non-linearly to the sensing diaphragm. These requirements greatly increase their cost and decrease production yields. A need therefore exists for improved differential pressure transducers of the type that employ isolation diaphragms. This need exists whether deflection is sensed by capacitive, inductive or other means.
Prior patents of the present inventor may be referred to as evidencing the state of the art relative to capacitive pressure transducers using ceramic sensing diaphragms (U.S. Pat. No. 4,295,376) and differential pressure transducers having a grounding structure for preventing excessive deflection of a sensing diaphragm (U.S. Pat. No. 4,458,537).
One current approach toward overcoming these problems is to use a "floating cell" configuration that transfers oil in parallel with the movement of the sensing diaphragm. Such structures, however, are expensive and difficult to manufacture, and so complex that they introduce additional performance problems.